1. Field of Invention
This invention relates to electronic circuits using phase-locked loops, and more particularly to a long term response enhancement for a digital phase-locked loop.
2. Description of Prior Art
A phase-locked loop is a device which produces an output signal at a frequency identical to and in phase with a signal supplied as an input thereto. As the input signal change in frequency, the output signal will likewise change in frequency in such a manner to maintain a zero difference in phase between the input and output signals.
In the past, phase-locked loops have been implemented using analog techniques. A basic analog phase-locked loop consists of three fundamental parts: a Phase Detector, a Filter, and a Voltage-Controlled Oscillator.
The Phase Detector is a device which detects the difference in phase between two input signals, and produces an output signal proportional to the amount of said phase difference. In a phase-locked loop, the two inputs to the Phase Detector are the Input Signal to the phase-locked loop, and the Output Signal of the phase-locked loop from a Voltage-Controlled Oscillator. The Output Signal from the Phase Detector is an analog signal, the magnitude of which is representative of the amount of phase difference between the two input signals thereto, hereinafter referred to as an error signal.
A Filter serves to remove any high-frequency components from the error signal produced by the Phase Detector, and provides a slowly varying Output Signal which is representative of the average error in phase between the Output Signal and the Input Signal.
A Voltage-Controlled Oscillator is an oscillator whose frequency is controlled by an Input Voltage. In a phase-locked loop, the Input Voltage to the Voltage-Controlled Oscillator is the output of the Filter, and is an error signal representative of the difference in phase between the Input Signal and the Output Signal. Due to the feedback of the Output Signal to one input of the Phase Detector, the frequency of the Voltage-Controlled Oscillator is adjusted by Input Voltage to produce agreement in phase between the Input Signal and the Output Signal.
While prior art implementations of phase-locked loops have employed varying amounts of digital techniques, analog techniques have always been present, to some extent; most notably in the filter section of the phase-locked loop.
The present digital phase-locked loop is distinguished from such prior art in that the implementation is accomplished using entirely digital techniques, as is the implementation of the present invention, a long term response enhancement for a digital phase-locked loop.